Wireless mobile communication technology uses various standards and protocols to transmit data between a node (e.g., a transmission station) and a wireless device (e.g., a mobile device). Some wireless devices communicate using orthogonal frequency-division multiple access (OFDMA) in a downlink (DL) transmission and single carrier frequency division multiple access (SC-FDMA) in an uplink (UL) transmission. Standards and protocols that use orthogonal frequency-division multiplexing (OFDM) for signal transmission include the third generation partnership project (3GPP) long term evolution (LTE) and LTE-Advanced (LTE-A).
In 3GPP radio access network (RAN) LTE and LTE-A systems, the node can be a combination of Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Node Bs (also commonly denoted as evolved Node Bs, enhanced Node Bs, eNodeBs, or eNBs) and Radio Network Controllers (RNCs), which communicates with the wireless device, known as a user equipment (UE). Examples of a UE include a mobile terminal, a tablet computer, a personal digital assistant (PDA) and a machine-type communication (MTC) device. The downlink (DL) transmission can be a communication from the node (or eNodeB) to the wireless device (or UE), and the uplink (UL) transmission can be a communication from the wireless device to the node. Instead of communication via eNodeBs, communication between wireless equipment can be performed using peer-to-peer or device-to-device communication.
As mobile technology advances, there is a requirement to provide accommodate progressively increasing demands for use of the wireless spectrum due to increasing user numbers and individual user demand for increased data throughput.
Carrier aggregation allows a single wireless connection to use multiple radio frequency (RF) carriers, known as Component Carriers (CCs) and increases channel bandwidth so that peak and average throughput can be increased. LTE Release 10 version defines signaling to support up to five component carriers to give a maximum combined channel bandwidth of up to 100 MHz. Component carriers can be intra-band contiguous, intra-band non-contiguous or even located in different bands (inter-band non-contiguous). Carrier aggregation is applicable to both uplink and downlink directions and to both Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
Carrier aggregation is considered as one of the main approaches to increase data rate performance of LTE-A systems and beyond. There are a large number of Release-10 UEs already available on the market that support aggregation of two carriers. It is expected that demands of carrier aggregation (CA) capable UEs with aggregation of multiple carriers in the future will become even higher. For example, LTE with Licensed-Assisted Access (LAA), which is also known as LTE for unlicensed spectrum or “LTE LAA”, may operate a with large number component carriers that may be aggregated at the UE to increase the peak data rate. Examples of unlicensed frequencies that could be utilized for carrier aggregation are 5 GHz, 2.4 GHz and 5150-5350 MHz. Unlicensed spectrum may include any RF spectrum that is contentiously accessed by electronic devices of the wireless communication network. By way of contrast, licensed spectrum (carriers) are non-contentiously accessed.
Conventionally, the LTE/LTE-A system utilizes licensed spectrum to operate. However, due to the increased user data rate demand over wireless and the explosive mobile traffic growth, industry is converging fast to utilize unlicensed spectrum for supplemental downlink and uplink capacity of an LTE system. To that end, a new study item, “Study on Licensed Assisted Access (LAA) using LTE”, has been approved by the 3rd Generation Partnership Project (3GPP) in RAN #65 meeting. LAA will feature Carrier Aggregation (CA) mechanism to aggregate a primary cell (PCell) or primary carrier, using licensed spectrum, to transmit critical information that requires quality of service and to control handover between cells, and a secondary cell (SCell) or secondary carrier, using unlicensed spectrum, for best effort data. LAA also studies the coexistence with other wireless technologies and the conformance to the regulatory requirements in unlicensed spectrum. The primary carrier and secondary carrier(s) may be denoted component carriers, as is conventional in carrier aggregation.
There is a requirement to provide a channel reservation mechanism in LTE LAA that takes account of the coexistence between different operators and different wireless technologies such as WiFi, all potentially competing for contentious access to the unlicensed spectrum. It is known to apply “listen-before-talk” criteria, which relies upon establishing carrier availability by instantaneous sensing of the RF medium, to mediate access to unlicensed carriers in LTE LAA. It is known in WiFi to employ CSMA/CA for contentious access to the medium and to use an RTS/CTS handshaking procedure to reduce the impact of hidden nodes when a point-to-point WiFi connection is established. In previously proposed LTE LAA systems, there is a preference that wireless connection establishment on LTE LAA should be implemented on the primary carrier (licensed spectrum) to ensure robustness. However, this can result in latency due to predetermined timing constraints imposed in LTE between, for example, an eNB scheduling request being sent and a response being received from the UE. Thus there is a perceived need for a lower latency mechanism for establishing an unlicensed spectrum connection and to make more efficient use of unlicensed spectrum in LTE LAA.